The role of NMDA and non-NMDA excitatory amino acid receptors in the excitation of primate spinothalamic tract neurons by mechanical, chemical, thermal, and electrical stimuli.

The role of excitatory amino acids (EAAs) in the excitation of monkey spinothalamic tract (STT) neurons following activation of cutaneous primary afferent fibers by noxious and non-noxious stimuli was investigated. The responses of STT neurons to either NMDA or non-NMDA EAA ligands were blocked by infusion of specific antagonists through a microdialysis fiber into the region surrounding the cells. Our results show that blockade of non-NMDA receptors results in a nearly complete elimination of the responses of STT neurons to all stimuli. Blockade of NMDA receptors results in an attenuation of the responses to noxious stimuli but, in addition, prevents the development of the sensitization of STT neurons that is often observed after intradermal injection of capsaicin. These observations further support a role of EAAs in the transmission of sensory information from primary afferent fibers to dorsal horn neurons and a role for NMDA receptors in the generation of hyperalgesia.     

Enhanced responses of spinothalamic tract neurons to excitatory amino acids accompany capsaicin-induced sensitization in the monkey.

Sensitization of the responses of dorsal horn neurons to mechanical stimulation may play a role in the generation of hyperalgesia. Intradermal injection of capsaicin (CAP) provides a model of experimental hyperalgesia that possesses a component of allodynia. This hyperalgesia is produced by chemical stimulation of C-fibers, leading to sensitization of dorsal horn neurons, including spinothalamic tract (STT) cells. The changes in the physiological responses of STT neurons following intradermal CAP in monkeys parallel the acute pain and hyperalgesia produced by intradermal CAP in humans. The present study addresses the role that excitatory amino acids (EAAs) may play in the sensitization of STT neurons by intradermal CAP. Our results show that the background discharge rate and the responses of STT cells to mechanical stimulation increase following intradermal CAP. In addition, the responses of the sensitized cells to one or more iontophoretically released EAA agonists, including NMDA, glutamate, aspartate, kainate, DL-alpha-amino-3-hydroxy-5-methyl-isoxazoleproprionic acid, and/or quisqualate, increase following intradermal CAP. It is proposed that an increase in the responses of STT neurons to EAAs contributes to the hyperalgesia produced by this noxious chemical stimulus.     

Protective Role of Nerve Growth Factor against Excitatory Amino Acid Injury during Neostriatal Cholinergic Neurons Postnatal Development

The interaction between excitatory amino acids (EAAs) and nerve growth factor (NGF) levels were studied on neostriatal cholinergic neurons during postnatal development. Striatal choline acetyltransferase (ChAT) activity and NGF levels were determined 7 days following EAA injection in 7-, 15-, 21-, 30-, and 50-day-old rats. ChAT activity was decreased 7 days after kainate (KA), quinolinate (QUIN), or quisqualate (QUIS) lesion. The reduction was most pronounced in 30-day-old rats. KA injection produced the greatest decrease in ChAT activity. Conversely, KA did not change NGF levels. QUIN and QUIS increased NGF protein and these effects were maximal with lesions in 21-day-old rats. In order to further characterize the effect of EAAs on NGF levels and ChAT activity, the time-course of the lesion was studied. We used 30-day-old rats as the maximal sensitivity of cholinergic neurons to EAAs was observed at this age. ChAT activity decreased 2 days following QUIN or QUIS injection and 1 day after KA. The EAA agonists also changed NGF levels. QUIN induced an increase in NGF levels 1 day after lesion. This effect was maintained to the last time point examined. In contrast, KA and QUIS induced transient increases in NGF levels that were only detected 2 and 4 days after injection, respectively. To study whether NGF is able to regulate EAA excitotoxicity on striatal cholinergic neurons, we studied ChAT activity 7 days after simultaneous injection of NGF plus QUIN, KA, or QUIS. Intrastriatal injection of exogenous NGF was able to block the decrease in ChAT activity observed following EAA injection alone. In conclusion, our results show that striatal cholinergic neurons have different vulnerabilities to excitotoxicity induced by EAAs during development. ChAT activity was decreased and NGF was increased by EAAs. However, those EAAs (QUIN and QUIS) that increased NGF had less effect on ChAT activity than KA which had little effect on NGF levels, suggesting that an increase in endogenous NGF levels by these agents may decrease their toxicity. This was confirmed by our finding that exogenous NGF protects cholinergic neurons against excitotoxic lesion. The combined results suggest that sensitivity to EAAs and the regulation of NGF may be crucial to the development of striatal cholinergic neurons.     

Effects of L-cysteine-sulphinate and L-aspartate, mixed excitatory amino acid agonists, on the membrane potential of cat caudate neurons

Responses evoked by L-cysteine-sulphinate (L-CSA) and L-aspartate (L-Asp) were recorded with intracellular electrodes from caudate neurons in halothane anesthetized cats. L-CSA and L-Asp were applied microiontophoretically to caudate cells and their effects on membrane and action potentials, as well as on cortically evoked synaptic potentials were evaluated. L-CSA and L-Asp induced depolarizations accompanied by regular firing resembling kainate (KA)- or quisqualate (QUIS)-induced excitation patterns (type 1) in 82% and 72% of the recorded neurons, respectively, and a mixed pattern consisting of a N-methyl-D-aspartate (NMDA)-like excitation (type 2) followed by a regular type 1 pattern in the remaining cells. In about a quarter of the cells the effects of L-CSA and L-Asp, but not those of KA or QUIS, were partially antagonized by 2-amino-7-phosphonoheptanoate (AP-7), a specific NMDA receptor antagonist. Kynurenate, a broad spectrum excitatory amino acid antagonist, blocked responses elicited by either L-CSA or QUIS. The actions of L-CSA and L-Asp on the firing pattern and membrane potential of cat caudate neurons in situ provide evidence in favor of their mixed agonist nature with respect to NMDA and non-NMDA excitatory amino acid receptors.     

Effects ofl-cysteine-sulphinate andl-aspartate, mixed excitatory amino acid agonists, on the membrane potential of cat caudate neurons

Responses evoked byl-cysteine-sulphinate (l-CSA) andl-aspartate (l-Asp) were recorded with intracellular electrodes from caudate neurons in halothane anesthetized cats.l-CSA andl-Asp were applied microiontophoretically to caudate cells and their effects on membrane and action potentials, as well as on cortically evoked synaptic potentials were evaluated.l-CSA andl-Asp induced depolarizations accompanied by regular firing resembling kainate (KA)- or quisqualate (QUIS)-induced excitation patterns (type 1) in 82% and 72% of the recorded neurons, respectively, and a mixed pattern consisting of aN-methyl-d-aspartate (NMDA)-like excitation (type 2) followed by a regular type 1 pattern in the remaining cells. In about a quarter of the cells the effects ofl-CSA andl-Asp, but not those of KA or QUIS, were partially antagonized by 2-amino-7-phosphonoheptanoate (AP-7), a specific NMDA receptor antagonist. Kynurenate, a broad spectrum excitatory amino acid antagonist, blocked responses elicited by eitherl-CSA or QUIS. The actions ofl-CSA andl-Asp on the firing pattern and membrane potential of cat caudate neurons in situ provide evidence in favor of their mixed agonist nature with respect to NMDA and non-NMDA excitatory amino acid receptors.     

Selective vulnerability of spinal cord motor neurons to non-NMDA toxicity

We previously reported that alpha-motor neurons in organotypic cultures of rat spinal cord (OTC-SC) are resistant to excitotoxicity induced through NMDA receptors. Here we describe the effects of non-NMDA glutamate receptor agonists kainic acid (KA) and quisqualic acid (QUIS) on motor neurons in OTC-SC. Large ventral horn acetylcholinesterase-positive neurons (VHANs), most of which are motor neurons, were quite sensitive to QUIS and KA toxicity and displayed losses of 95% and 94%, respectively. Small VHANs were reduced by 41% and 61% only. Identical results were obtained in cultures stained for non-phosphorylated neurofilaments. These observations demonstrate that alpha-motor neurons are considerably more sensitive to KA and QUIS than to NMDA toxicity. The proposed excitotoxic mechanism of ALS, therefore, is most likely mediated through non-NMDA glutamate receptors.     

Development of excitatory amino acid induced cytotoxicity in cultured neurons

The neurotoxicity of the excitatory amino acids (EAAs) L-glutamate (L-glu), L-aspartate (L-asp), N-methyl-D-aspartate (NMDA), kainate (KA), quisqualate (QA) and RS-alpha-amino-3-hydroxy-5-methyl-4-isoxazolopropionate (AMPA) was followed as a function of development in primary cultures of cerebral cortex neurons and cerebellar granule cells. These two types of neurons express, respectively, glutamate receptor subtypes with sensitivity to all of these excitatory amino acids or only to glutamate and aspartate. None of the EAAs were toxic in cerebral cortex neurons at 2 days in culture, whereas at culture day 4 the neurons became sensitive to glutamate, at day 5 to KA followed by sensitivity to QA at day 6, and finally to NMDA, L-asp and AMPA at day 7. The rank order of potency of the EAAs was in cerebral cortex neurons cultured for 12 days: L-asp (ED50 = 0.5 microM) = L-glu (ED50 = 1 microM) greater than AMPA (ED50 = 10 microM) greater than NMDA (ED50 = 65 microM) greater than QA = KA (ED50 = 100 microM). Cerebellar granule cells were insensitive to all of the EAAs at 3 and 5 days in culture but at day 8 the cells became sensitive to toxicity induced by L-glu (ED50 = 70 microM) and L-asp (ED50 = 30 microM). In order to determine ED50 values for L-asp and L-glu accurately, media in these experiments also contained 500 microM of the glutamate uptake inhibitor L-aspartate-beta-hydroxamate.(ABSTRACT TRUNCATED AT 250 WORDS)     

Excitatory Amino Acids Modulate Phosphoinositide Signal Transduction in Human Epileptic Neocortex

Stimulation of phosphoinositide (PI) hydrolysis by norepinephrine (NE), carbachol (Carb), and excitatory amino acids (EAAs) was measured in slices prepared from neocortex excised during epilepsy surgery. NE and Carb markedly enhanced PI turnover (EC50: NE, 12 microM; Carb, 661 microM) as reflected by [3H]inositol monophosphate (IP1) accumulation in tissue slices prelabeled with [3H]myoinositol. These effects were dose-dependent, saturable, and five to six times higher than basal IP1 accumulation. A weaker stimulation (twofold) was observed with quisqualate (QUIS; EC50, 1.1 microM) and glutamate (GLU; EC50, greater than 1 mM), while minimal or no stimulation was seen with kainate (KA) and N-methyl-D-aspartate (NMDA). Agonist-stimulated PI turnover was significantly reduced in samples from actively spiking epileptic neocortex versus nonspiking areas as defined by electrocorticography (NE, -23%, p less than 0.05; Carb, -44%, p less than 0.01). Preincubation of slices with various EEAs inhibited Carb-induced IP1 formation. The maximal extent of inhibition (1 mM) was both amino acid-dependent (IC50: NMDA, 5 microM; KA, 3.3 microM; QUIS, 47 microM; GLU, greater than 1 mM). These data suggest that epileptic activity modulates PI metabolism and alters receptor-effector coupling. As important mediators of epileptogenesis, EAAs may interfere++ with the efficiency of this second messenger system.     

Comparison of the spinal neuropathic pain induced by intraspinal injection of N-methyl-d-aspartate and quisquate in rats

Objective

Excitatory amino acids play important roles in the development of secondary pathology following spinal cord injury (SCI). This study was designed to evaluate morphological changes in the dorsal horn of the spinal cord and assess profiles of pain behaviors following intraspinal injection of N-methyl-D-aspartate (NMDA) or quisqualate (QUIS) in rats.

Methods

Forty male Sprague-Dawley rats were randomized into three groups : a sham, and two experimental groups receiving injections of 125 mM NMDA or QUIS into their spinal dorsal horn. Following injection, hypersensitivity to cold and mechanical stimuli, and excessive grooming behaviors were assessed serially for four weeks. At the end of survival periods, morphological changes in the spinal cord were evaluated.

Results

Cold allodynia was developed in both the NMDA and QUIS groups, which was significantly higher in the QUIS group than in the NMDA group. The mechanical threshold for the ipsilateral hind paw in both QUIS and NMDA groups was significantly lower than that in the control group. The number of groomers was significantly higher in the NMDA group than in the QUIS group. The size of the neck region of the spinal dorsal horn, but not the superficial layer, was significantly smaller in the NMDA and QUIS groups than in the control group.

Conclusion

Intraspinal injection of NMDA or QUIS can be used as an excitotoxic model of SCI for further research on spinal neuropathic pain.     

Activation of locus coeruleus neurons by nucleus paragigantocellularis or noxious sensory stimulation is mediated by intracoerulear excitatory amino acid neurotransmission

The nucleus paragigantocellularis (PGi), located in the rostral ventrolateral medulla, is one of two major afferents to the nucleus locus coeruleus (LC). Electrical stimulation of PGi exerts a robust, predominantly excitatory influence on LC neurons that is blocked by intracerebroventricular (i.c.v.) administration of the broad spectrum excitatory amino acid (EAA) antagonists kynurenic acid (KYN) or gamma-D-glutamylglycine (DGG), but not by the selective N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-7-phosphonoheptanoate (AP7). I.c.v. injection of KYN or DGG also blocked activation of LC neurons evoked by noxious somatosensory stimuli. These results indicate that activation of LC neurons by PGi and noxious stimuli may be mediated by an EAA acting at a non-NMDA receptor in LC. In the present study, microiontophoretic techniques were used to determine the sensitivity of LC neurons in vivo to the selective EAA receptor agonists kainate (KA), NMDA and quisqualate (QUIS). Microinfusion and microiontophoresis were also used to determine whether direct application of KYN, the preferential non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3 dione (CNQX) or the selective NMDA receptor antagonist 2-amino-5-phosphonovalerate (AP5) onto LC neurons blocked excitation elicited by stimulation of PGi or the sciatic nerve. The results demonstrated that individual LC neurons were robustly activated by direct application of KA, NMDA and QUIS. Iontophoretically applied KYN reduced or completely antagonized responses evoked by all 3 agonists. In contrast, iontophoretically applied AP5 strongly attenuated NMDA-evoked excitation, while KA-and QUIS-evoked responses were not affected by this agent. Furthermore, direct application of KYN or the specific non-NMDA receptor antagonist, CNQX, onto LC neurons substantially attenuated or completely blocked synaptic activation produced by PGi or sciatic nerve stimulation in nearly every LC neuron tested. Microinfusion of the selective NMDA receptor antagonist AP5 had no effect on sciatic nerve-evoked responses. These results confirm our hypothesis that activation of LC neurons from PGi is mediated by an EAA operating primarily at a non-NMDA receptor subtype on LC neurons. Furthermore, these findings provide additional support for the hypothesis that this pathway mediates at least some sensory-evoked responses of LC neurons.     

Enhanced phosphorylation of NMDA receptor 1 subunits in spinal cord dorsal horn and spinothalamic tract neurons after intradermal injection of capsaicin in rats.

The functional enhancement of NMDA receptors after peripheral tissue injury is proposed to contribute to the sensitization of spinothalamic tract (STT) cells and hyperalgesia. Protein phosphorylation is a major mechanism for the regulation of NMDA receptor function. In this study, Western blots, immunofluorescence double labeling, and the retrograde tracing method were used to examine whether phosphorylation of NMDA receptor 1 (NR1) subunits increases in spinal cord tissue and spinal dorsal horn neurons, especially in STT cells, after injection of capsaicin (CAP) into the glabrous skin of one hindpaw of anesthetized rats. Western blots showed that phosphorylated NR1 protein in spinal cord tissue was increased 30 min after CAP injection. Immunofluorescence double-labeling staining showed no significant difference in the number of the NR1-like immunoreactive neurons in laminae I-VII in the lumbosacral segments (L(4)-S(1)) on the ipsilateral and the contralateral sides 30 min after CAP or vehicle injection. However, the numbers of phospho-NR1-like immunoreactive neurons were significantly increased on the ipsilateral side compared with the vehicle injection group. STT cells were labeled by bilateral microinjections of the retrograde tracer fluorogold into the lateral thalamus, including the ventral-posterior lateral nucleus. Immunofluorescence staining was performed at 30, 60, and 120 min after CAP injection or at 30 min after vehicle injection. There was a significant increase in the proportion of STT cells with phosphorylated NR1 subunits compared either with the contralateral side 30 and 60 min after CAP injection or either side of animals after intradermal injection of vehicle. These results provide direct evidence that NMDA receptors in STT cells are phosphorylated after CAP injection.     

NMDA antagonists attenuate hypertension induced by carotid clamping in the rostral ventrolateral medulla of rats

The purpose of these experiments were to study the interactions of N-methyl-D-aspartate (NMDA) with baroreceptor reflexes induced by transient carotid clamping. Adult male Sprague-Dawley rats were anesthetized with urethane. Bilateral common carotid artery occlusion resulted in a reversible and reproducible hypertension in the vagotomized animals. This hypertensive reaction was blocked by intraventricular injection of NMDA antagonists, such as 2-amino-7-phosphono-heptaneoate (AP-7) and phencyclidine (PCP). We also found that blood pressure-sensitive neurons of the rostral ventrolateral medulla (RVLM) could be classified into two groups, on the basis of their responses to norepinephrine given intravenously. Using pressure microejection and single unit recording, we observed that clamping of the common carotids resulted in excitation of type I neurons. This evoked excitation, similar to that induced by NMDA, was blocked by locally applied AP-7. However, the carotid occlusion-induced responses of type II neurons were not blocked by AP-7. In conclusion, the present data suggest that NMDA receptors are involved in hypertensive responses during carotid occlusion, perhaps involving a site in the rostral ventrolateral medulla.     

Selective excitotoxic pathology in the rat hippocampus

The pattern of cell loss and neuronal degeneration resulting from multiple microinjections of N -methyl-D-aspartate (NMDA), ibotenate (IBO), quisqualate (QUIS), and kainate (KA) into hippocampus was studied, together with the protection provided by the NMDA antagonist 3-(±)-2-carboxypiperazin-4-yl-propyl-l-phosphonate (CPP). Histological evaluation was carried out after 7 days of survival. NMDA and IBO resulted in an extensive loss of all cells in the hippocampus including dentate gyrus, hilar cells, and CA3-CA1 pyramidal cells, but there was an absence of damage to areas and structures outside hippocampus. After QUIS and KA injections the hippocampal damage was limited to hilar cells in the dentate gyrus, CA3 pyramidal cells, and partial loss of CA1 cells; there was extensive extrahippocampal damage including entorhinal cortex, amygdala, layers III, V, and VI of ventral neocortex, olfactory areas, and various thalamic nuclei. CPP provided almost complete protection from the effects of intrahippocampal injections of NMDA and IBO, but did not affect the hippocampal cell loss found after QUIS and KA (with the exception of minor protection of some CA1 cells). CPP protected most extrahippocampal sites from the damage resulting from QUIS and KA, indicating that such excitotoxic cell death is indirect and involves NMDA receptor activation by an endogenous agent. The use of multiple microinjections as opposed to single injections allows a clearer interpretation of selective excitotoxic vulnerability.     

NMDA receptors in nucleus accumbens modulate stress-induced dopamine release in nucleus accumbens and ventral tegmental area

Converging evidence suggests that dopamine (DA) transmission in nucleus accumbens (NAcc) is modulated locally by an excitatory amino acid (EAA)-containing input possibly originating in medial prefrontal cortex (PFC). In the present study, we examined the effects of intra-NAcc administration of EAA receptor antagonists on stress-induced increases of NAcc DA levels and of dendritically released DA in the ventral tegmental area (VTA). Local injection of the NMDA receptor antagonist—AP-5 (0.05, 0.5, and 5.0 nmoles)—dose-dependently potentiated increases in NAcc DA levels elicited by 15 min of restraint stress. In contrast, local application of equivalent doses of the kainate/AMPA receptor antagonist—DNQX—failed to alter the NAcc DA stress response reliably. In a separate experiment, we found that intra-NAcc injection of AP-5 also potentiated stress-induced increases in VTA DA levels. These results indicate that EAAs acting at NMDA receptors in NAcc can modulate stress-induced DA release in this region. Our data indicate, however, that this action exerts an inhibitory influence on the NAcc DA stress response, suggesting that the relevant population of NMDA receptors are not located on NAcc DA terminals. The fact that intra-NAcc AP-5 injections also potentiated the DA stress response in VTA suggests instead an action mediated by NMDA receptors located on NAcc neurons that feedback, directly or indirectly, to cell bodies of the mesocorticolimbic DA system. Synapse 26:225–234, 1997. © 1997 Wiley-Liss Inc.     

Antagonists of synaptic and amino acid excitation of neurones in the cat spinal cord

In the spinal cord of the anaesthetized cat microelectrophoretically administered (+/-)-cis-2,3-piperidine dicarboxylate (2,3-PDA), (+/-)-cis-2,5-piperidine dicarboxylate (2,5-PDA), gamma-D-glutamylglycine (gamma DGG), beta-D-aspartyl-beta-alanine (beta DAA), (+/-)-2-amino-4-phosphonobutyrate (2-APB), (+/-)-2-amino-5-phosphonovalerate (2-APV) and (+/-)-2-amino-7-phosphonoheptanoate (2-APH) were assessed as antagonists of chemical excitation of dorsal horn interneurones and Renshaw cells by N-methyl-D-aspartate (NMDA), L-aspartate, quisqualate (QUIS), kainate and L-glutamate, and of monosynaptic and polysynaptic excitation by impulses in primary afferent fibres of muscle and cutaneous origin. Whereas polysynaptic excitation of interneurones was readily and reversibly depressed by 2-APV, 2-APH, beta DAA, gamma DGG and 2,3-PDA, all of which also reduced excitation by NMDA (and L-aspartate) more than that by QUIS (and L-glutamate), no selective antagonism of monosynaptic excitation could be demonstrated. In particular, 2,3-PDA, which depressed excitation by kainate to a greater extent than that by either QUIS or NMDA, appeared to have no effect on monosynaptic excitation. The results support the involvement of L-aspartate as the transmitter of some spinal excitatory interneurones, but none of the antagonists tested were considered suitable for assessing the role of L-glutamate as the transmitter of some spinal primary afferent fibres.     

Amino acid neurotransmission between fimbria-fornix fibers and neurons in the lateral septum of the rat: a microiontophoretic study

We investigated the nature of the excitatory amino acid and the type of amino acid receptor involved in the projection of fimbria-fornix (fi-fx) fibers on neurons in the lateral septal complex (LSC) of the rat. It appeared that neurons which were strongly orthodromically activated (SOA) by stimulation of fi-fx fibers were excited by glutamate (GLU) and aspartate (ASP) at much lower ejecting currents than neurons which were only weakly orthodromically excited. In addition, GLU was a stronger agent than ASP, particularly in SOA septal cells. Two amino acid antagonists tested, glutamic acid diethylester (GDEE) and 2-amino-5-phosphonovaleric acid (2-APV), selectively antagonized responses to the amino acid agonists quisqualate (QUIS) and N-methyl-D-aspartate (NMDA), respectively. They also depressed GLU- and ASP-induced responses, although in that case the antagonists frequently had to be expelled with currents higher than those needed to block QUIS- and NMDA-evoked excitations. Furthermore, GDEE frequently antagonized GLU-induced responses better than ASP-evoked excitations, whereas 2-APV often blocked responses to ASP more effectively than those to GLU. It was observed that GDEE, ejected with currents that blocked responses to QUIS reversibly, decreased the number of synaptic responses induced in SOA cells by fi-fx stimuli. Synaptically induced excitation in these neurons was consistently unaffected by 2-APV, even when the antagonist was expelled with high currents. According to these results, LSC neurons, in particular the SOA neurons, are more readily activated by GLU than by ASP. Monosynaptic excitations elicited in SOA septal cells by fi-fx stimuli appear to be predominantly if not exclusively mediated by QUIS receptors. There are indications that GLU-induced responses in the LSC neurons are presumably mediated by the QUIS receptors. From these data it may be inferred that GLU rather than ASP is the transmitter involved in the projection of fi-fx fibers on LSC neurons.     

Distinct modulatory roles of sigma receptor subtypes on glutamatergic responses in the dorsal hippocampus

Sigma ligands have been previously shown to modulate the N-methyl-D-aspartate (NMDA) response in the dorsal hippocampus, such that low doses of sigma agonists dose-dependently potentiate the response. Recent studies with the sigma ligand 4-IBP found it to act differently from the sigma ligands (+)-pentazocine and DTG in the modulation of 5-HT firing activity in the dorsal raphe nucleus (DRN), as its effects were not blocked by the sigma antagonists which reversed those of (+)-pentazocine or DTG. Thus, this study set out to characterize 4-IBP's action at sigma receptors using the hippocampal paradigm of sigma ligand activity. Interestingly, we found that in 50% of the neurons recorded, 4-IBP (20 microg/kg i.v.) produced a potentiation of both NMDA- and quisqualate (QUIS)-induced responses. In the other 50% of neurons, 4-IBP produced an attenuation of both QUIS and NMDA responses. The sigma1 antagonist NE-100 blocked the reduction induced by 4-IBP, while the nonselective sigma antagonist haloperidol blocked all responses induced by 4-IBP. These data suggest that, in this model, 4-IBP may be acting as an agonist or inverse agonist of sigma receptors. Furthermore, the initial responses to NMDA and QUIS were higher in the group in which 4-IBP induced an attenuation of the firing activity. This suggests a modulatory role for 4-IBP on glutamatergic neurotransmission in the hippocampus, which appears to involve two distinct pathways, mediated by different sigma1 receptor subtypes, an NE-100 and haloperidol-sensitive sigma1 receptor, and an NE-100-insensitive, haloperidol-sensitive sigma1 receptor. This modulatory role has implications for disorders that involve glutamatergic transmission in the hippocampus.     

Epileptiform bursts elicited in CA3 hippocampal neurons by a variety of convulsants are not blocked by N-methyl-d-aspartate antagonists

Intracellular and extracellular recordings from CA₃ hippocampal neurons in vitro were used to study the ability of several NMDA (N-methyl-d-aspartate) receptor antagonists to suppress epileptiform bursts induced by NMDA and convulsants not thought to act at NMDA receptors. The antagonists, APV (d-2-amino-5-phosphonovalerate), AP-7 (d,l-2-amino-7-phosphonoheptanoate) and CPP (d,l-3-[(±)-2-car☐ypiperazin-4-yl-]-propyl-1-phosphonic acid), blocked the spontaneous and evoked bursts induced by NMDA. CPP, but not APV or AP-7, prevented the development of bursts induced by Mg-free medium. The NMDA antagonists failed to block bursting induced by kainate, 7 mM K⁺, mast cell degranulating peptide, anoxia or spontaneous bursting. In some cases the NMDA antagonists induced spontaneous bursts or enhanced burst frequency, a proconvulsant effect. It is concluded that activation of NMDA receptors is sufficient but not necessary for burst generation in the CA₃ region.     

Bidirectional modulation of windup by NMDA receptors in the rat spinal trigeminal nucleus

Activation of afferent nociceptive pathways is subject to activity-dependent plasticity, which may manifest as windup, a progressive increase in the response of dorsal horn nociceptive neurons to repeated stimuli. At the cellular level, N-methyl-d-aspartate (NMDA) receptor activation by glutamate released from nociceptive C-afferent terminals is currently thought to generate windup. Most of the wide dynamic range nociceptive neurons that display windup, however, do not receive direct C-fibre input. It is thus unknown where the NMDA mechanisms for windup operate. Here, using the Sprague-Dawley rat trigeminal system as a model, we anatomically identify a subpopulation of interneurons that relay nociceptive information from the superficial dorsal horn where C-fibres terminate, to downstream wide dynamic range nociceptive neurons. Using in vivo electrophysiological recordings, we show that at the end of this pathway, windup was reduced (24 +/- 6%, n = 7) by the NMDA receptor antagonist AP-5 (2.0 fmol) and enhanced (62 +/- 19%, n = 12) by NMDA (1 nmol). In contrast, microinjections of AP-5 (1.0 fmol) within the superficial laminae increased windup (83 +/- 44%, n = 9), whereas NMDA dose dependently decreased windup (n = 19).These results indicate that NMDA receptor function at the segmental level depends on their precise location in nociceptive neural networks. While some NMDA receptors actually amplify pain information, the new evidence for NMDA dependent inhibition of windup we show here indicates that, simultaneously, others act in the opposite direction. Working together, the two mechanisms may provide a fine tuning of gain in pain.     

The contribution of excitatory amino acids to central sensitization and persistent nociception after formalin-induced tissue injury.

The contribution of excitatory amino acids (EAAs) to the development of central sensitization and persistent nociception in response to tissue injury in rats was examined following the subcutaneous injection of formalin into the hindpaw. Formalin-induced nociceptive behaviors were enhanced by intrathecal pretreatment with the EAAs L-glutamate and L-aspartate. An enhancement of the formalin nociceptive response was also produced by intrathecal pretreatment with the receptor-selective EAA agonists NMDA and trans-(+/- )-1-amino-1,3-cyclopentane dicarboxylic acid (ACPD), but not (R,S)-alpha-amino-3-hydroxy-5-methylisozazole-4-propionic acid hydrobromide (AMPA). The effect of NMDA was enhanced by a combined administration with AMPA or APCD. Formalin nociceptive responses were dose-dependently reduced by intrathecal pretreatment with the NMDA receptor antagonists 2-amino-5-phosphonovaleric acid (APV) and (+)-MK-801 hydrogen maleate, but not the selective AMPA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione or the selective metabotropic EAA receptor antagonist 2-amino-3-phosphonopropionic acid. The results suggest that EAAs acting at the NMDA receptor contribute to central sensitization and persistent nociception following subcutaneous formalin injection.